With that being said, I'd really like to encourage the OP to store his data base (and boot his instance) from persistent disk. Running any database on scratch disk (without replication) is probably not a good idea. Even with hourly backups (make sure you are testing restores!) you still stand to have up to an hour of data loss and the pain of doing the restore if your instance should fail.
In addition, when using PD for all block storage you can start with a smaller instance. If you need more horsepower you can terminate that instance and boot from a larger instance with a minimal amount of downtime.